Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. A receiver is an electronic device which receives and processes a wireless electromagnetic signal. A transmitter and receiver may be combined into a single device called a transceiver.
Transmitters, receivers, and transceivers often include components known as oscillators. An oscillator may serve many functions in a transmitter, receiver, and/or transceiver, including generating a local oscillator signal (usually in a radio-frequency range) for upconverting baseband signals onto a radio-frequency (RF) carrier and performing modulation for transmission of signals, and/or for downconverting RF signals to baseband signals and performing demodulation of received signals. Such oscillators may include components known as phase-locked loops (PLLs). A PLL may be a control system configured to generate an output signal whose phase is related to the phase of the input “reference” signal. A phase-locked loop circuit may compare the phase of the input signal with a phase signal derived from its output oscillator signal and adjusts the frequency of its oscillator to keep the phases matched.
In order to compare the phase of the input reference signal with an output oscillator signal, a PLL may include a component known as a phase detector. A phase detector as used in a PLL may be an electronic device that generates a signal (typically a voltage signal) which represents the difference in phase between the input reference signal and the output signal. For a Type I PLL, its phase detector is often implemented as a logical exclusive OR (XOR) gate, with the input reference signal and the output signal as inputs to the XOR gate. Accordingly, the output of the XOR gate phase detector may have an output of “high” or logic 1 when the input reference signal and output oscillator signal have different values, and may have an output of “low” of logic 0 when the input reference signal and output oscillator signal have the same values.
One shortcoming of the traditional XOR gate phase detector is the small lock range of PLLs utilizing XOR gate phase detectors. A lock range is the range of phase differences between the input reference signal and output oscillator signal for which the PLL can match or “lock” the phases of input reference signal and output oscillator signal. In addition, the output of the XOR gate phase detector may have a positive or negative slope, as shown in FIG. 6. Such negative or positive slope may cause positive feedback in a PLL depending on the sign of the PLL loop, thus potentially leading to instability. For instance, the PLL may have a metastable point which may lead to excessive peaking in the lock time of the PLL if the initial phase between the input reference signal and output oscillator signal at startup of the PLL is at this metastable point.
Various approaches have been used to address these shortcomings, including replacing the XOR gate phase detector with a Type II phase detector (also known as a charge pump-based phase detector), and/or increasing PLL bandwidth. However, such approaches may be undesirable. For example, increasing PLL bandwidth may increase the number of required reference spurs, and a charge pump-based detector may introduce undesirable noise. A time-to-digital converter may also be used as a phase detector to overcome the shortcomings of an XOR gate phase detector, but would likely generate more noise and consume more power, as well as requiring greater design complexity and circuit area.